I.单项选择从每小题所给的A、B、C三个选项中，选出可以填入空白处的正确答案。（每小题1分，满分15分）
1. —If you help us solve the problems, you may go to America for a free trip.
—It’s a very nice _____________, but I really can’t accept it.
A. reason B. excuse C. offer
2. Mr. Wang is a nice teacher. He often helps his students with _____________ physics after school.
A. them B. their C. theirs
